Cold ignition and hot re-ignition of high intensity discharge (HID) lamps, particularly ceramic metal halide (CMH) lamps, have very different operational characteristics and requirements. In order to start a cold CMH lamp, a ballast needs to deliver high voltage pulses (e.g., about 2000 volts) to ionize the gas to initiate the arc discharge between the electrodes. Immediately following initiation of the arc discharge, or breakdown, the lamp enters a glow mode, during which the lamp voltage is still relatively high, e.g., on the order of a few hundred volts. Subsequently, the lamp transitions from the glow mode to an arc mode, during which the arc warms up to reach its steady-state discharge voltage, e.g., on the order of 100 volts.
In order to re-ignite, or restrike, a thermally hot lamp, much more energy is required; thus, the breakdown voltage is much higher, e.g., greater than 10,000 volts. During hot restrike, if there is no protection circuit, the self-oscillating ballast can continuously deliver excessively high voltage pulses to the lamp until the electrical stresses exceed the limitations of the ballast, leading to destruction of critical semiconductor components.
Accordingly, it is desirable to provide robust protection circuitry for self-oscillating lamp ballasts for protecting ballast components during hot re-ignition, or restrike, of the lamp.
A hot restrike protection (HRP) circuit provides shut down protection for a self-oscillating high intensity discharge (HID) lamp ballast of the type comprising a pair of complementary switching devices in a bridge configuration with a gate drive inductor in series with a second, or control, inductor at the junction between the switching devices. The HRP circuit effectively comprises a three-terminal device for coupling across the control inductor. In particular, the HRP circuit comprises a sensing network for sensing voltage across the control inductor, a breakdown network for providing a breakdown path upon reaching a predetermined restrike voltage threshold across the control inductor, and a shutdown network for shutting down operation of the ballast until the lamp is sufficiently cool for restarting, thereby protecting ballast components during hot restrike, or re-ignition, of the lamp.